Continuous boron nitride nanotube fibers

ABSTRACT

Described herein are apparatus, systems, and methods for the continuous production of BNNT fibers, BNNT strands and BNNT initial yarns having few defects and good alignment. BNNTs may be formed by thermally exciting a boron feedstock in a chamber in the presence of pressurized nitrogen. BNNTs are encouraged to self-assemble into aligned BNNT fibers in a growth zone, and form BNNT strands and BNNT initial yarns, through various combinations of nitrogen gas flow direction and velocities, heat source distribution, temperature gradients, and chamber geometries.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/305,994, filed Oct. 21, 2016, which is a U.S. National Stage ofInternational Patent Application No. PCT/US15/027,570, filed Apr. 24,2015, which claims the benefit of U.S. provisional patent applicationNo. 61/983,674, filed Apr. 24, 2014; U.S. provisional patent applicationNo. 62/101,318, filed Jan. 1, 2015; and U.S. provisional patentapplication No. 62/101,323, filed Jan. 8, 2015, each of which isincorporated by reference in its entirety.

STATEMENT REGARDING GOVERNMENT SUPPORT

None.

FIELD

The present disclosure generally relates to the continuous formation ofboron nitride nanotube (BNNT) fibers, BNNT strands, and BNNT yarns.

BACKGROUND

Generally, BNNT structures may be formed by thermally exciting a boronfeedstock in a chamber in the presence of nitrogen gas at an elevatedpressure. Unlike carbon nanotubes (CNTs), U.S. Pat. No. 8,206,674 toSmith et al., incorporated by reference in its entirety, indicates thatBNNTs form without the presence of chemical catalysts, and preferably atelevated pressures of about 2 atm to about 250 atm. CNTs, on the otherhand, typically require the presence of chemical catalysts such as metalcatalysts. It has been shown that BNNTs do not form in the presence ofsuch catalysts, indicating that the formation of BNNTs is fundamentallydifferent than the formation of CNTs.

Most contemporary BNNT synthesis methods have severe shortcomings,including one or more of having low yield, short tubes, discontinuousproduction, poor crystallinity (i.e., many defects in molecularstructure), and poor alignment. Although there is no agreed uponstandard in the scientific literature, the term ‘high quality’ BNNTsgenerally refers to long, flexible, molecules with few defects in thecrystalline structure of the molecule. However, there are no knownreports of the growth of continuous BNNT fibers or BNNT strands,particularly having few defects and good alignment. The BNNT “streamers”described in U.S. Pat. No. 8,206,674 to Smith et al., for example, formnear a nucleation site such as the surface of the boron feedstock, butwere limited to about 1 cm in length. BNNT “streamers” at such lengthsare inadequate for producing BNNT yarns.

What is needed are apparatus, systems, and methods for the continuousproduction of BNNT fibers and BNNT strands, having few defects and goodalignment.

BRIEF SUMMARY

This disclosure relates to apparatus, systems, and methods for thecontinuous production of BNNT fibers, BNNT strands and BNNT initialyarns, having few defects and good alignment. BNNTs may be formed bythermally exciting a boron feedstock in a chamber in the presence ofpressurized nitrogen. BNNTs are encouraged to self-assemble into BNNTfibers in space near the boron feedstock, through various combinationsof nitrogen gas flow direction and velocities, heat source distribution,temperature gradients, and chamber geometries.

BNNT fibers and BNNT strands may form BNNT initial yarns, which may bedry spun to form dry spun BNNT initial yarns. This disclosure describesapparatus and methods for forming and collecting BNNT yarns, andcontrolling various properties of the BNNT yarns. BNNT initial yarn maybe collected and used to form BNNT yarn. BNNTs, BNNT fibers, BNNTstrands, BNNT initial yarns and BNNT yarns have numerous potentialapplications. For instance, they may be used as heat transfer mediums,such as in the materials going into a diode, light-emitting diode,transistor or integrated circuit. They may also be used for addingstructural strength and other advantageous properties to fabrics andcomposites such as ceramic, metal matrix and polymer based composites.

Embodiments may comprise a process for producing aligned BNNT fibers inwhich a boron feedstock is thermally excited in a chamber, such as by atleast a laser; nitrogen gas is fed into the chamber at an elevatedpressure; a heat distribution profile is established to form a BNNTgrowth zone in the chamber; and a shear force profile is established inthe chamber to align BNNTs produced in and emerging from the growthzone. The process may proceed without catalysts in some embodiments,i.e. with no reactive feedstock other than boron and nitrogen. In someembodiments, thermally exciting the boron feedstock may form a boronmelt. In some embodiments, establishing a shear force profile alignsBNNTs in an alignment direction, and nitrogen gas may be fed to thechamber in a direction that is generally aligned with alignmentdirection.

Generally, the BNNT growth zone is downstream of the boron feedstock,and represents the self-assembly region for initial BNNT structures. Insome embodiments, a BNNT interference zone may be formed in the chamberdownstream of the BNNT growth zone. An interference zone is a region inthe chamber in which BNNTs become long enough to interfere with eachother and form BNNT strands. Some embodiments may further includeforming a BNNT stretch zone in the chamber downstream of the BNNTinterference zone. In the stretch zone, BNNT strand structures stretchin length and decrease in diameter, and may form BNNT initial yarn.

In some embodiments, establishing a shear force profile in the chamberto align BNNTs produced in and emerging from the growth zone may includeestablishing a velocity distribution of the nitrogen gas in a planeperpendicular to the flow of the nitrogen gas that generates a desiredlevel of BNNT alignment. A velocity distribution of the nitrogen gas maybe established by adjusting one or more parameters, such as: setting afan speed, establishing a heat distribution profile that generates aconvective buoyancy about the boron feedstock, manipulating nitrogen gaspressure, increasing the volumetric flow rate of nitrogen gas in aportion of the chamber, cooling the nitrogen gas, and adjusting aninsertion rate of nitrogen gas into the chamber. A nitrogen gas velocitydistribution may be established in the interference zone, and in a planeperpendicular to the flow of the nitrogen gas, that generates a desiredlevel of BNNT alignment.

In some embodiments, the volumetric flow rate of nitrogen gas in aportion of the chamber may be increased, such as by reducing across-sectional area of a portion of the chamber, introducing cowlinginto a portion of the chamber, and positioning a damper in a portion ofthe nitrogen gas flow.

In some embodiments, establishing a shear force profile in the chambermay include establishing a heat distribution profile in the BNNT growthzone that produces aligned BNNTs emerging from the growth zone.

In some embodiments, a heat distribution profile of heat provided to theboron feedstock may be established that forms a BNNT self-assembly fluxdistribution that generates a desired level of BNNT alignment.

In some embodiments, BNNT initial yarn may be formed downstream of thegrowth zone. BNNT initial yarn may be collected in some embodiments,such as by spooling the initial yarn as it emerges from a stretch zone.

In some embodiments, a coating may be applied to all or to a portion ofthe aligned BNNT fibers. In some embodiments, BNNT initial yarn may betwisted, braided, and/or woven.

Embodiments of an apparatus for producing aligned boron nitride nanotube(BNNT) fibers may include a chamber providing a boron feedstock mountingsurface; a heat source for thermally exciting a boron feedstock on themounting structure, wherein the heat source provides heat into thechamber; a nitrogen gas supply system configured to feed nitrogen gas tothe chamber upstream of the mounting structure and in a directiongenerally aligned with a BNNT formation direction; and a growth zoneregion downstream of the mounting structure. In some embodiments, theheat source may be configured to establish a heat distribution profileto form a BNNT growth zone in the growth zone region. Some embodimentsmay include an interference zone region downstream of the grown zoneregion. In some embodiments, the interference zone region may have areduced cross-sectional area, configured to increase the volumetric flowrate of nitrogen through the interference zone region. Some embodimentsmay include a stretch zone region downstream of the interference zoneregion where the cross-sectional area may be adjusted to assisting thestretching of the BNNT initial yarn.

In some embodiments, the heat source may be configured to thermallyexcite a portion of the boron feedstock to form a BNNT self-assemblyflux distribution that generates a desired level of BNNT alignment. Someembodiments may include a fan upstream of the mounting structure andconfigured to control the velocity distribution of the nitrogen gas in aplane perpendicular to the flow of the nitrogen gas.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show example pressurized nitrogen gas distribution andBNNT self-assembly flux distribution profiles.

FIGS. 2A-2C show BNNT alignment enhancements in different zones.

FIG. 3 is a drawing of an early prototype apparatus for producingcontinuous BNNT fibers.

FIG. 4 shows a series of five still images extracted from experimentalvideo data using a prototype apparatus as described with respect to FIG.1.

FIG. 5 is a diagram of an embodiment of an apparatus for formingcontinuous BNNT fibers, strands and initial yarns.

FIG. 6 is a diagram of another embodiment of an apparatus for formingcontinuous BNNT fibers, strands and initial yarns.

FIGS. 7A-7E illustrate several cross-sections of the apparatus in FIG.6, taken in a plane perpendicular to the material flow direction.

FIGS. 8A-8D illustrate the BNNT structures in several cross-sections ofthe apparatus in FIG. 7, taken in planes parallel and perpendicular tothe material flow direction.

FIGS. 9A, 9B, and 9C are 2.3×1.7 mm images of BNNTs produced byembodiments of the apparatus and methods described herein.

FIGS. 10A-10C illustrate the spooling of BNNT initial yarn according toembodiments described herein.

FIGS. 11A-11E show drawings of various BNNT yarns.

FIGS. 12A-12C show various BNNT yarns with coatings.

FIG. 13 is an image of a BNNT self-assembly region in a prototypeembodiment, feeding into a BNNT “cotton ball” of BNNT fibers and someBNNT strands.

FIG. 14 shows BNNT initial yarn acquired on a variable diameter spool.

FIG. 15 shows BNNT initial yarn acquired on a cylindrical spool.

DESCRIPTION

The following description is of the best currently contemplated mode ofcarrying out exemplary embodiments of the present approach. Thedescription is not to be taken in a limiting sense, and is made merelyfor the purpose of illustrating the general principles of the invention.

Described herein are processes and apparatus for producing continuousfibers and strands of BNNT, and in particular BNNT fibers and BNNTstrands having good fiber alignment, as well as BNNT yarns. Generally,BNNTs may be formed in a self-assembly region under nitrogen gas. BNNTfiber shear alignment may be enhanced by making adjustments to thevelocity distribution of the nitrogen gas, and/or by making adjustmentsto the BNNT self-assembly region flux distribution, as discussed below.

The formation of BNNTs is fundamentally different than the formation ofCNTs. Synthesis of CNTs typically requires a chemical catalyst, such asa metal catalyst. Unlike the production of CNTs, embodiments forproducing BNNT fibers may be catalyst-free. Although gaseous diatomicnitrogen is difficult to crack, it can readily dissolve into moltenmetals and silicates to form compounds such as silicon nitride. Becauseof the enhanced synthesis at elevated pressures, the inventorshypothesize that the elevated pressure significantly increases thesupersaturation of dissolved nitrogen into boron micro-droplets thatform over the thermally excited boron feedstock. The inventorshypothesize that the elevated pressure increases nitrogen dissolution inboron micro-droplets, which in turn causes supersaturation of boronnitride. BNNTs vigorously self-assemble from boron nitride, and possiblyother precursors, in a region in close proximity to the boronmicro-droplets.

Thus, BNNT synthesis at elevated pressures proceeds without a catalyst.CNTs, on the other hand, typically require a metal catalyst. Forexample, as evident in chemical vapor deposition of methane gas overiron droplets (among other processes), CNT synthesis requiressupersaturating dissolved carbon on a catalytic metal droplet orcluster. As a result, the metal catalyst must be removed through asubsequent purification step. Catalyst-free BNNT synthesis, on the otherhand, does not require a similar purification step. In fact, because thereactants are only boron from the feedstock and nitrogen gas, in someembodiments the boron may be nearly completely consumed duringsynthesis.

Generally, BNNT fibers may be generated by thermally exciting a boronfeedstock in the presence of nitrogen gas at an elevated pressure. Ithas been determined that feeding nitrogen gas at an elevated pressuredrives the synthesis of BNNTs and results in higher production rates andimproved BNNT quality and yield. In some embodiments, the boron andnitrogen reactants are maintained at an elevated pressure, such as anelevated pressure of at least 2 atmospheres to about 250 atmospheres,or, for example, an elevated pressure of at least 2 atmospheres to about12 atmospheres. Boron-containing feedstock may include, as examples,elemental boron, elemental boron powder, boron nitride, boron nitridepowder, cubic boron nitride powder, and hexagonal boron nitride powder.

Thermal sources such as laser heating and plasma heating have been shownto produce BNNT. For example, as described above, U.S. Pat. No.8,206,674 to Smith et al., the contents of which are incorporated byreference in their entirety, describes a process for preparing largequantities of BNNTs through laser heating at elevated pressures (e.g.,about 2 atm to about 250 atm). In the process disclosed in this patent,boron vapor is created through thermally exciting a boron source andnitrogen gas near the boron source with a laser or other heat source;mixing the boron vapor with nitrogen gas at an elevated pressure, andharvesting boron nitride nanotubes formed at a nucleation site. Theprocess disclosed in that patent resulted in the formation of BNNTsaround nucleation site including the surface of the boron feedstock. Theshort BNNT “streamers” were limited to around 1 cm in length, anddifficult to harvest from the region of the nucleation site withoutlosing alignment. Unlike that patent, embodiments of the present methodmay be configured to produce and extract continuous BNNT fibers, BNNTstrands and BNNT initial yarns, including fibers, strands, and initialyarns of substantial length.

U.S. Pat. No. 8,753,578 to Smith et al. discloses a laser-driven processfor producing BNNTs. This patent teaches thermally exciting an end of aboron-containing rod in a chamber with nitrogen gas at an elevatedpressure, forming BNNTs along a cooled condenser positioned in closeproximity to the end of the boron-containing rod. The cooled condenserrotates over a collection tube that removes BNNT from the condenserthrough a vacuum. Under this method, the BNNTs form on the condenser inone direction, and are extracted in another direction. Thus, BNNT“streamer's” lengths may be limited by many features of the describedprocess, including condenser rotation speed, along with self-assemblylimitations from temperature gradients. Also, the extraction hindersachieving good BNNT fiber, strand and initial yarn alignment.

Other means of thermal excitation may be used. For example, U.S. patentapplication Ser. No. 14/529,485 to Smith et al., the contents of whichare incorporated by reference in their entirety, describes a process forpreparing large quantities of BNNTs through inductively-coupled plasma(ICP).

Prior apparatus and methods for producing BNNTs relied on condensers inclose proximity to the boron feedstock to induce nanotube formation. Inembodiments of the present approach, BNNTs are encouraged toself-assemble into BNNT fibers in space near the boron feedstock,through various combinations of nitrogen gas flow direction andvelocities, heat distributions, temperature gradients, and chambergeometries. One or more BNNT strands may form when multiple lengths ofBNNT fibers commonly align and/or aggregate and have someinterconnection between fibers. The term BNNT initial yarn, as usedherein, refers to BNNT fibers and BNNT strands that form both in anddownstream of BNNT formation in the self-assembly region. BNNT initialyarn may include various admixtures of particles of boron, BN (boronnitride), and h-BN (hexagonal-BN), depending on the production method.

Generally, under the present approach, a boron melt is generated byheating a boron feedstock and adjacent nitrogen gas in a high pressurenitrogen environment. Embodiments of the present approach may beperformed in an apparatus having a chamber, through which nitrogen gasat elevated pressures may flow. The high pressure nitrogen gas flowingacross the boron melt may have a flow velocity distribution (generallyreferenced as Vcoflow). FIG. 1 shows a demonstrative general nitrogengas flow velocity distribution, for a boron melt positioned in thecenter of the horizontal axis. The nitrogen gas flow velocitydistribution will depend on several factors, including for example,chamber geometry and any physical obstructions, nitrogen compression andinjection rate, and the convection buoyancy effect from the heat source.Vcoflow may be generated and controlled by various methods. For example,in embodiments, Vcoflow may be controlled by one or more of a pressuredifference created by a flow fan, the injection rate of compressednitrogen gas, the initial pressure of the compressed nitrogen gas, andheat driven convection or buoyancy, applied across a flow controlstructures including baffles.

Some embodiments may include a flow fan, such as a flow fan upstream ofthe boron feedstock. A flow fan can be utilized to vary the velocity ofthe nitrogen gas flowing to the boron melt. The fan creates a pressuredifference by raising the pressure of the nitrogen gas.

Nitrogen gas injection rate and initial pressure may be used to adjustVcoflow. For example, nitrogen gas can be injected into the flow to varythe velocity of the nitrogen gas, vary the velocity distribution of thenitrogen gas, vary the pressure of the nitrogen gas, and vary themagnitude of Vcoflow. One of ordinary skill should appreciate that oneor more parameters may be adjusted to modify Vcoflow, and may establisha Vcoflow for an apparatus that generates the desired alignment in thegenerated BNNT fibers and strands.

One of ordinary skill should also appreciate that amount anddistribution of heat may vary parameters that impact the alignment inBNNT fibers and strands. For example, the amount of heat and thedistribution of heat applied to the boron melt, and also the nitrogengas in the region of the boron melt, can be utilized to vary thenitrogen gas convection forces, such as buoyance. In turn, nitrogen gasconvection forces may be used to vary the velocity and velocitydistribution of the nitrogen gas Vcoflow.

FIG. 1 also shows a general BNNT self-assembly region flux distribution.The boron melt can have varying temperatures in different regions of themelt. The rate of boron vaporization, the rate of nitrogen gasdissolving into the boron and the rate of boron vapor and precursormaterial feeding the BNNT self-assembly process depends on the detailsof the boron melt temperature and nitrogen gas Vcoflow. By varying thedistribution of the heat going into the boron melt, the fluxdistribution of material feeding the BNNT self-assembly process can bevaried. One of ordinary skill should appreciate that these parametersmay be optimized for an apparatus, to produce BNNT fibers, BNNT strands,and BNNT initial yarns having a desired alignment. Apparatus may varysignificantly, and therefore parameters for one apparatus may notgenerate the same results in a separate apparatus.

In embodiments of the present approach, there are generally threeprimary zones in the BNNT fiber, strand and initial yarn formation. Aswill be described herein, shearing forces and conditions may be used ineach zone to align BNNT structures. Zones may overlap to varying degreesin different embodiments, i.e. there are no sharp boundaries betweenzones, and one zone may penetrate deep into a subsequent zone. Zone 1 isthe “growth zone,” and is generally the self-assembly region for initialBNNT structures. In the growth zone, BNNT molecules referred to as BNNTfibers grow from the isolated micro droplets and the boron melt and fromthe boron melt itself. For example, heating a boron feedstock in a highpressure nitrogen environment causes micro-droplets to condense abovethe boron melt in what this disclosure refers to as the “growth zone” ofthe self-assembly region. At sufficiently high temperature, such as atemperature that causes boron droplets to form, as would be known in theart, BNNTs and BNNT fibers form from the micro-droplets in the growthzone.

In embodiments of the present approach, BNNT fibers may acquirepreferred alignment in the growth zone. FIG. 2(A) shows BNNT fiber 21 ina growth zone changing alignment due to the nitrogen gas, Vcoflow, flow.The alignment is generally a result of the nitrogen gas differentialvelocity distribution across the self-assembling BNNT fibers and isinfluenced by the flux density of the self-assembling BNNT fibers. Ifthe velocity distribution were flat, there would likely be no preferredalignment direction for the BNNT fibers. If the nitrogen gas velocitydistribution were turbulent, the alignment would be localized withcharacteristics of the length scales of the turbulent flows. One ofordinary skill should appreciate that the Vcoflow nitrogen gasdifferential velocity generating the ideal alignment will vary betweenapparatus, and therefore various nitrogen gas differential velocitiesshould be evaluated for the impact on alignment in the growth zone.

Zone 2 is the “interference zone” in which BNNTs become long enough tointerfere with each other and form BNNT strands. In the interferencezone, BNNT fibers may cohere by intermolecular forces, creating a loose,but generally interconnected cloud or moving stream of cotton-like BNNTstructures. In most embodiments, a cotton-like state of BNNT firstoccurs in this interference zone as the BNNT strands assemble. FIG. 2(B)shows an enhancement in the alignment of BNNT strands 22 in aninterference zone due the velocity distribution of the Vcoflow nitrogengas. The alignment is self-reinforcing, e.g., the forces between BNNTfibers are greater if they are close to each other over their lengths,i.e. aligned, and they tend to enhance the alignment set up by the shearforces from the velocity distribution and intern this brings the BNNTstrands closer together.

Under the present approach, BNNT strands may acquire preferred alignmentand/or further alignment enhancement in the interference zone. In theinterference zone, the nitrogen gas differential velocity distributionacross the interacting BNNT strands may align some BNNT strands that didnot achieve the desired alignment in the growth zone, and also enhancethe alignment of BNNT strands forming the cotton-like cloud of BNNTs. Ifthe nitrogen gas velocity distribution were flat in this region therewould be clumping of the BNNTs but alignment would only be present ifthere were alignment already present from Zone 1. If the nitrogen gasvelocity distribution were turbulent, the alignment would be localizedwith characteristics of the length scales of the turbulent flows. One ofordinary skill should appreciate that the nitrogen gas differentialvelocity generating the ideal alignment will vary between apparatus, andtherefore various nitrogen gas differential velocities should beevaluated for the impact on alignment in the interference zone.

Zone 3 is the “stretch zone” in which BNNT strand structures stretch inlength and decreases in diameter. FIG. 2(C) shows BNNT structures 23,which comprise BNNT initial yarns (and possibly BNNT fibers, BNNTstrands and other species), improving alignment due to the overallinitial yarn diameter decreasing in the stretch zone, among othershearing forces present in the zone. In the stretch zone, the strainingforce is generally resisted by aerodynamic drag and, in someembodiments, gravity. In some embodiments, BNNT structures entering thestretch zone will include BNNT initial yarns. In some embodiments, BNNTfibers and BNNT strands may begin to form BNNT initial yarns in thestretch zone. It should be appreciated that the primary zones mayoverlap by varying degrees in different embodiments. For example, in oneembodiment there may be a discernable transition between zones, whereasin another embodiment there may be a gradual transition between zones.

Under the present approach, the stretch zone may include a BNNTcollection device such as a take-up real or spool. The take-up reel orspool may be used to pull on the cotton-like BNNT stream of BNNT fibersand BNNT strands to create longitudinal strain and resulting shearalignment in the stretch zone. The amount of shear can be controlled tosome extent by the difference in velocity between V1 and V2, where V1 isthe natural velocity due to convection buoyancy and Vcoflow, and V2 isthe physical velocity induced by the take-up reel or spool. The velocitydifference for alignment is dependent on the particular apparatus used.One of ordinary skill should appreciate that these parameters may bevaried for a particular apparatus to achieve the desired alignment.

Under the present approach, the combination of the velocity distributionshear forces in Zone 1 and Zone 2, as well as any structural changes inthe BNNT structures in Zone 3 (generally, the structures will includeBNNT fibers, BNNT strands and BNNT initial yarn) as the initial yarn isstretched and reduced in diameter, combine to create the overall BNNTalignment as seen in the BNNT initial yarn wound on the take upreel/spool.

Generally, an apparatus for producing continuous BNNT fibers may includea chamber, a boron feedstock holder, a pressurized source of nitrogengas, a nitrogen gas injection mechanism, a flow fan, a heat source,apparatus regions containing the growth zone, interference zone, andstretch zone, and a collection region. Some embodiments may include aflow control lining. A flow control lining may be structure thatprovides a cross section area and shape for the nitrogen gas flow intothe chamber, or through a region of the chamber, to affect the nitrogengas velocity distribution For example, embodiments may include a flowcontrol lining upstream of the boron feedstock, but downstream of thenitrogen gas injection point(s) for the chamber. The nitrogen gas flowelements and boron feedstock holder may be configured for thermalexcitation from the heat source, and for exposure to high pressurenitrogen gas flow. In some embodiments of the present approach,boron-containing feedstock may positioned in a chamber in which thermalexcitement of the boron-containing feedstock may be provided in a firstdirection, and the pressurized nitrogen gas may be provided in a seconddirection. In some embodiments, the first direction and the seconddirection may be generally parallel. In some embodiments, the firstdirection and the second direction may be generally perpendicular. Oneof ordinary skill should appreciate that other relative directions maybe employed as desired, and typically will depend on the geometry of aparticular apparatus, the type and mounting of feedstock, and thedesired collection method.

FIG. 3 is a drawing of an early prototype apparatus for producingcontinuous BNNT fibers, BNNT strands and BNNT initial yarns. Theapparatus in FIG. 3 includes a chamber 32 in which boron feedstock 37 ispositioned on a holder. Boron feedstock 37 is thermally excited from aheat source 36, which in this prototype is a laser, at a temperaturesufficient to create a boron melt as described above. Pressurizednitrogen gas 38 flows across the boron melt 37. In the prototypeapparatus, the pressurized nitrogen gas flow 38 is in a direction thatis generally perpendicular to the direction of the heat 36 supplied tothe chamber. It should be understood that the relative orientations ofthe heat and nitrogen gas may vary in different embodiments.

In FIG. 3, BNNT precursors 31 form as boron micro-droplets condenseabove the surface of the boron melt 37, and as pressurized nitrogen gas38 begins to dissolve in the boron micro-droplets. In this particularembodiment, BNNT precursors 31 are continuously produced and flow frombottom to top (as shown in the drawing, and with the understanding thatthe actual direction depends on the apparatus' orientation), and BNNTinitial yarn 310 is continuously produced and flows top to bottom. Asthey travel upward, BNNT precursors form BNNT fibers, also 31. BNNTstrands 39 form along the way to the top of the figure, and the BNNTinitial yarn forms as the BNNT strands come together and are pulled andstretched downwards towards the boron melt. By focusing the heat source36 on roughly the center of mass of the boron melt 37, the heat source36 in this particular embodiment induced the counter-flow of BNNTinitial yarn 310 as shown in FIG. 3. It should be appreciated that therelative material flows may be altered by varying the distribution ofthe heat source 36 on different regions of the boron melt 37. Inexperiments performed using a prototype of this embodiment, the maximumvelocity of the BNNT precursors 1 was about 1 m/s, and the velocity ofthe drawn BNNT fiber 310 was about 0.25 m/s. The BNNT initial yarn asdescribed below, may be anchored to a liquid or solid draw bar 35. Theshearing action at the interfaces 33 and their associated nitrogen gasvelocity distributions 38, as described above, induces alignment in thegrowing BNNT initial yarn 310.

In the embodiment shown in FIG. 3, the growth zone (Zone 1) 31 feedsinto the interference zone (Zone 2) 39, where in this embodiment theBNNT initial yarn begins to form and is looping back, and then feedsinto stretch zone (Zone 3) 310 feeding the initial yarn to the liquiddraw 35.

Aligned counter-flowing BNNT initial yarn produced in the prototypeembodiment shown in FIG. 3 was extracted during experimental runs by: 1)manipulating the BNNT flux distribution via manipulating the input heatdistribution going into the boron melt, 2) manipulating the heatdistribution going into the adjacent nitrogen gas, and 3) manipulatingthe pressurized nitrogen gas velocity distribution. The heat going intothe boron melt was adjusted to enhance the BNNT flux above the center ofthe boron melt and heat going into the convection buoyancy wasredistributed to make the nitrogen gas flow more laminar. Thesevariables were adjusted manually until an acceptable/desired alignmentof the BNNT initial yarn was achieved.

FIG. 4 shows a series of five still images extracted from experimentalvideo data using a prototype apparatus as described with respect to FIG.3. The BNNT initial yarn 42, as indicated by the label in the top image,leaves the video frame to the left of the image. A series of verticalarrows on the image sequence denote a particle 41 that has beenentrained in the growing BNNT initial yarn 42. The particle 41 serves asa convenient reference to indicate the motion and velocity of the fiber42. The width of each image represents about a 1.25 inch field-of-view.The images are spaced approximately 50 msec apart in time (total elapsedtime for the sequence is 200 ms).

The prototype apparatus shown in FIG. 5 is one example of an apparatusfor producing continuous BNNT fibers, strands and initial yarns. Itshould be appreciated that numerous other embodiments may be madewithout departing from the principles described here. For example, theheat source 513 may be focused on upper regions of the boron melt 51 toinduce continuous material flow in the same direction as the nitrogengas flow, Vcoflow 514.

FIG. 5 is a diagram of an embodiment of an apparatus 50 in whichcontinuous BNNT fibers, strands and initial yarns 510, 511, and 512 formwithout a counter-flow. In apparatus 50, a boron melt 51 is formed byheating 513 a boron feedstock on a target holder 52. Pressurizednitrogen 53 is supplied to the chamber from nitrogen chamber 50 frombelow. Some embodiments may include control fan 54 for controllingpressure changes and convection buoyancy inside apparatus 50. Forinstance, control fan 54 may also expand the temperature gradient aboveboron melt 51 to further enhance BNNT formation. BNNT fibers 510self-assemble above the boron melt 51, and due to the nitrogen gas flow,convection buoyancy, and flow control cowling 55, self-assemble in thegrowth zone 56. The distribution of heat 513 on the boron melt and intothe adjacent nitrogen gas 51, as discussed above, controls thedistribution of BNNT flux above the boron melt 51. It should be notedthat the growth zone 56 is generally identified for simplicity; it couldextend from just above the boron melt 51, and overlap with interferencezone 57. FIG. 5 includes arrows 53 to show the general pressurizednitrogen gas velocity distribution profile. It should be observed thatvarious chamber geometries, such as cowling 55, may be used to controland modify the nitrogen gas velocity profile, Vcoflow, 514 along thelength of other embodiments. For instance, the apparatus profile at aparticular zone may be reduced to intensify shearing along that region,and enhance alignment. However, one or ordinary skill should appreciatethat over-shear can be undesirable as can be under-shear, and thus itmay be necessary to experimentally establish the shearing and nitrogengas profiles producing the desired alignment for an apparatus.

As BNNT fibers 510 enter the interference zone 57 (also generallyidentified), BNNT fibers interact with each other, coming in contact,and forming BNNT strands and initiating BNNT initial yarns 511 moving atvelocity V1. BNNT initial yarns 512 proceed into stretch zone 58, inwhich aligned BNNT fibers, BNNT strands and initial yarns 512 stretchdue to the difference between the velocity V2 of take-up reel or spool59. The apparatus in FIG. 5 shows various features and configurationsthat may be incorporated into an apparatus for producing continuous BNNTfibers, strands and initial yarns. It should be appreciated that otherembodiments may be made without departing from the principles describedherein.

Embodiments of the present approach may be used to form BNNT yarns.Generally, BNNT yarns may be formed by twisting and spooling BNNTinitial yarns, which are generally made of BNNT strands and BNNT fibers.The BNNT initial yarn may be collected on a rotating spool 59 inside achamber at a region downstream of BNNT formation. BNNT formation may bethrough a number of processes. As discussed above, high-temperature,high-pressure processes allows BNNT fibers and BNNT strands toself-assemble downstream of the boron feed source. The BNNT fibers andBNNT strands may continue to flow downstream of the self-assemblyregion, toward a collection region.

In some embodiments, a take-up reel or spool 59 may be used in thecollection region to spool the BNNT initial yarn. The process may becontinuous until a spool has reached its capacity to collect BNNTinitial yarn, at which time the spool is replaced with a new spool.Also, the spooled BNNT initial yarn may be re-spooled onto a new spoolto form BNNT yarn, and may be twisted during re-spooling to enhancevarious physical properties, and may be re-spooled with one or moreadditional BNNT initial yarns to form larger BNNT yarns. The BNNTinitial yarn may be composited with materials at any time starting fromwhen the BNNT initial yarn forms, to when it is spooled, to when it isspun into BNNT yarn, to when it is unwound and combined with compositingmaterials.

Apparatus for forming BNNT initial yarns may feature a chamber havingmultiple regions that define a material path. Depending on the methodused for forming BNNT fibers, the chamber may be configured to operateat enhanced pressures (e.g., up to 250 atm). The material path may besuch that nitrogen gas is re-circulated. The initial region may be forforming BNNTs, and may include a boron feed source such as a boron ball,and a nitrogen gas supply. The chamber may include a region downstreamof the initial region for BNNT fiber self-assembly. The apparatus mayinclude a distribution of heat sources to lengthen the self-assemblyregion and enhance the conversion of boron particles to BNNTs. Thechamber may include a region downstream of the self-assembly region forBNNT fibers to align and come into contact or bind, forming BNNT strandsand BNNT initial yarns. Such regions may have different geometries toencourage alignment, binding, and desired nitrogen gas and BNNT initialyarn speeds. The chamber may include a spooling region downstream of thebinding region to collect BNNT initial yarns on a spool. The spool maybe inside the apparatus, or may be external to the apparatus if asuitable pressure exchange system is available. The apparatus mayinclude de-spooling and re-spooling features, to allow BNNT initialyarns to be twisted and combined to form BNNT yarns.

FIG. 6 is a diagram of an embodiment of an apparatus 60 for forming BNNTinitial yarn 612 using a laser-driven, high-temperature, high-pressureprocess. It should be appreciated that alternative heat sources,including multiple lasers, may be used, as described above. Theapparatus 60 includes a chamber 630 designed for operating at elevatedpressures, that may in some embodiments go up to about 250 atm, anddefining an interior volume with multiple regions. In some embodiments,the chamber may be operated at elevated pressures of at least 2atmospheres to about 12 atmospheres. As described herein, thecross-section shape and area may vary at each region. In thelaser-driven, high-temperature, high-pressure process, a laser beamheats a boron melt positioned on target holder 62 in the chamber to forma boron melt 61. The laser also heats the nitrogen gas adjacent to theboron melt. Boron vapor and BNNT precursors evaporate from the boronmelt 61. A high pressure nitrogen gas flows from nitrogen source 63 andacross the boron melt 61, providing a nitrogen source for BNNTformation. Some embodiments may include a flow fan 64, as describedabove. BNNTs 610 form in the growth zone 66, which in some embodimentsmay include the hot plume region of boron vapor, other BNNT precursorsand hot nitrogen above the boron melt 61 surface.

BNNT fiber self-assembly occurs directly from the boron melt 61 and onboron droplets 66 or particles that condense from the boron vaporslightly above the boron melt 61 surface. Location A in FIG. 6 shows theapproximate region in which BNNT structures 610, including BNNT fibersand BNNT strands 611, form in the laser driven high-temperaturehigh-pressure process. Of course, additional steps may be required toform BNNT fibers and BNNT strands if another BNNT formation method isused.

The highly crystalline and catalyst-free BNNT fibers 611 produced insome embodiments of the present approach are typically 2-wall andfrequently 3-wall structures, although fibers may include between 1 walland up to about 10 walls. Typically, larger numbers of walls become lesscommon as the number of walls increases. In this production method, theonly process impurities are particles of boron and BN includingamorphous BN and h-BN.

In general, the BNNT fibers can grow from the droplets in the vaporregion in any direction; however there are two sources of preferentialdirection for BNNT fiber growth. First, BNNT fibers that grow directlyon the boron melt tend to grow in the direction of the nitrogen andvapor flow. In FIG. 6, the nitrogen gas flows upward, past the boronfeedstock 61, and through regions A, B, and so on. Apparatus 60 mayinclude cowling 65 and other internal structures to modify nitrogen flowvelocity profile distribution and vapor flow, to enhance alignment asdescribed above. Second, BNNT fibers 611 that grow from the boron vapordroplets tend to be aligned with the nitrogen gas flow by a combinationof shear alignment in the flow direction, and the fact that the locationof the BNNT fiber growth on the droplets is free to move into alignmentwith the direction of flow due to the shear alignment.

The BNNT fibers 611 in the interference region 67 are fairly closelyaligned in the nitrogen gas flow and can align with each other andenhance the attraction with each other. As a consequence, BNNT strandstend to form within the flow, and the final BNNT initial yarn 612 thatforms is a combination of the BNNT strands and BNNT fibers that may bemore randomly oriented relative to the BNNT strands. In some instancesthe boron droplets travel farther down the chamber, downstream oflocation A, before becoming super cooled and beginning to form BNNTfibers, BNNT strands, and BNNT initial yarns, in the stretch region 68.

In one embodiment of the apparatus, the BNNT fiber self-assembly region(including the BNNT initial yarn formation region) includes the chamberinterior beginning with the boron melt (or other boron feedstock) tomany centimeters above the boron ball. The precise region will depend onthe geometry of the chamber, particularly near location A, nitrogen gasflow rates, and heat sources.

As discussed above, additional energy may be supplied to the BNNTnitrogen gas adjacent to the boron melt, the boron melt and the fiberself-assembly by a secondary heat source, such as one or more additionallasers, etc. The additional energy from a secondary heat source mayextend the region of self-assembly of the BNNT fiber self-assembly.Extending the self-assembly region lengthens the BNNT fibers, andincreases the number and/or size of BNNT strands formed, as more of theboron vapor and boron droplets are converted to BNNT fibers.

Due to the high temperatures and high pressures involved in the BNNTfiber self-assembly and concurrent/subsequent BNNT strand and BNNTinitial yarn formation process, the BNNT yarn processes described hereinare largely different from processes applicable to the formation ofcarbon nanotube yarns, and low quality BNNT formation processes wherethe BNNTs typically are shorter, have a large number of walls, multipledefects, and are usually not very fibril.

The following paragraphs describe the flow management downstream of theboron source in the apparatus shown in FIG. 6. In experiments conductedusing a prototype apparatus, the vertical speed of the mixture of boronvapor, nitrogen gas, boron droplets, and BNNT initial yarn 612 typicallyranged between 0.1 m/s to about 5 m/s, depending on conditions includingthe size of the boron melt, the amount of heat source, the nitrogen gaspressure, the distribution of heat going into the nitrogen gasconvection buoyancy, and the geometry of the chamber immediatelydownstream of the boron ball and in the BNNT fiber self-assembly region.In some embodiments, the nitrogen gas flows in a chimney-likecirculation pattern, or in a loop, and the flows may be mostly laminar.In some embodiments, back-streaming and local turbulent flows mightoccur. The cross section of the chamber regions may be designed suchthat nitrogen gas velocity maintains a local BNNT initial yarn speed ata speed that keeps a slight tension on the BNNT initial yarn afterformation, i.e., at and downstream of the self-assembly region.

The overall nitrogen gas flow may controlled by one or more of: 1) theamount and distribution of heat introduced by the heat source, includingany secondary heat source, 2) the location and amount of the cooling invarious regions of the chamber, 3) the nitrogen pressure, and 4) thecross-sectional profiles of the different sections of the apparatusalong the nitrogen gas loop, including local baffles. FIGS. 7A-7Eillustrate demonstrative cross-section profiles (A)-(E) for theapparatus 60 shown in FIG. 6. The cross-sections shown in FIGS. 7A-7Eare taken at planes perpendicular to the flow direction through thechamber. The example cross-sections in FIGS. 7A-7E are generallycircular or ovular, but other embodiments may use different geometriesto manage the nitrogen gas flow speed and keep a slight tension in theBNNT initial yarn at and downstream of the self-assembly region.

In the prototype embodiment shown in FIGS. 6 and 7A-7E, location A isgenerally circular and large enough to convey the hot vapors upstreamfrom the boron source. However, it is generally preferable to avoidvapor speeds that create overly turbulent flows. In some embodiments, itmay be preferable to manage the cross sections and volume to allow downdraft currents within the chamber.

In the prototype embodiment, location B is similar to location A, but inother embodiments may be slightly smaller or larger to keep the slighttension in the BNNT initial yarn 612 as the drag from the viscosity ofthe BNNT fibers, BNNT strands, and BNNT initial yarn 611 changes as thetemperature of the vapor lowers downstream of the self-assembly region.

In the prototype embodiment, locations C and D include a more openprofile. The open profile allows the BNNT initial yarn 612 to maintainits speed downstream of the self-assembly region, and move nearer to theedge of the chamber interior in preparation for spooling prior tolocation E. The open profile allows the process to be demonstratedthough with less locational control of the BNNT initial yarn going ontothe spooler.

In the prototype embodiment, location E is a downdraft from the internalspooling operation, discussed below. In other embodiments, location Emay be downstream of the pressure chamber exterior walls, such as when amore open geometry is utilized. After spooling the BNNT initial yarn,the chamber cross-section may be most anything, provided that cooling ofthe nitrogen gas takes place and nitrogen is cycled back to thebeginning of the process, i.e., the region of the boron source, such asthe boron melt used in the demonstrated embodiments, at the desiredinitial nitrogen gas speed.

One of ordinary skill should appreciate that the exact shapes andcross-sectional areas of sections B, C, D and E can be modified inreal-time by dampers 71. A damper material, as commonly seen in achimney, may be placed in the flow path of the nitrogen gas but not inthe flow path of the BNNT initial fiber where the angle/cross section ofthe damper can be varied by changing its angle with respect to thedirection of the nitrogen gas flow. For example, the cross-sectionsshown in FIGS. 7A-7E include various dampers at different locations (thedampers are omitted from FIG. 6 for simplicity). The dampers manage theflow of nitrogen gas and thereby assist in managing the speed/flow ofthe nitrogen gas in contact with the BNNT initial yarn.

The following paragraphs describe the formation and handling of BNNTstrands and BNNT initial yarns. As the BNNT fibers form on and above theboron ball in the self-assembly region, such as the region near locationA inside the chamber shown in FIG. 6, at least some of the BNNT fibersbegin to commonly align and form BNNT strands. The BNNT strands in turnare largely aligned with the BNNT initial yarn. In experiments run usinga prototype embodiment, the BNNT strands are about 1 micron to about 50microns in diameter. Also, a BNNT initial yarn at any locationdownstream of the self-assembly region may have 1 BNNT strand to about30 BNNT strands. More BNNT strands may be possible, depending onoperating conditions and chamber design. In experiments run using aprototype embodiment, 2 BNNT strands to 20 BNNT strands represented themajority of the BNNT initial yarn produced.

The BNNT initial yarn diameter may change as the material moves fartherdownstream of the self-assembly region. In experiments run using aprototype embodiment, the BNNT initial yarns in and around location Bare typically about 0.05 mm to 4.0 mm in diameter, and the BNNT initialyarns in and around location C are about 0.02 mm to 1.5 mm in diameter.

As the BNNT initial yarn moves through the process, the BNNT strands maybind closely together. This binding results in part from the slighttension kept on the BNNT initial yarn as discussed above, and also fromthe alignment attractive forces, i.e. some alignment tends to produceforces that create more alignment due to the close proximity of multipleBNNT fibers with each other and their mutual attraction. As the BNNTsare electrically insulating while CNTs are electrically conductive orsemi-conductors, these BNNT alignment attractive forces are typicallymuch larger in BNNTs as compared with CNTs which is another example ofhow BNNT processes are significantly different from CNT processes.

Embodiments may include steps to prepare the BNNT initial yarn forspooling. For example, a step of changing the general cross-sectionalshape of the BNNT initial yarn may improve spooling. In experiments runusing a prototype embodiment, the shape of BNNT initial yarn at locationD may be compressed into an ovular or flat profile, in preparation forspooling. In experiments run using a prototype embodiment, compressedBNNT initial yarns had a general thickness of micron 1 to about 100microns, and a width of about 2 microns to about 300 microns. The BNNTstrands at location D are preserved in the structure and do becomecloser to each other. These ranges of values are dependent on thedetails of the embodiment and values outside of these ranges can beanticipated in other embodiments.

As another example, changing the direction of the BNNT initial yarn mayassist in spooling the initial yarn. For instance, as shown in FIG. 6,the angle of the BNNT initial yarn 612 at location D, relative to theflow direction from the self-assembly region, may be about 60 degrees.In the prototype embodiment, the angular change assisted the spoolingprocess. The angle in other embodiments may vary from 0 to 360 degrees,depending on the chamber interior geometries used for the process.Additionally, a longer initial yarn path to location D may be utilizedto enhance the viscous drag on the BNNT initial yarn, and therebyenhance stretching and BNNT fiber-to-fiber and strand-to-strandconnections as the material is wound onto the spool.

As described above, the BNNT initial yarn in some embodiments mayinclude less aligned components and randomly aligned components. Theless aligned or randomly aligned components of the BNNT initial yarn mayprovide separation of the BNNT strands.

FIG. 8 shows cross-sections of portions of the regions labeled A-D inFIG. 6, the cross-sections taken in planes perpendicular and parallel tothe material flow direction through the apparatus. Location A isgenerally the self-assembly region in which the BNNT fibers 82 grow frommicro-droplets 81 and BNNT strands form. Locations B and C are BNNTinitial yarn 83 transport regions, in which the BNNT initial yarn 83becomes slightly more compact transversely 83 a, and is steered towardthe spool. Location D is the spooling region. The BNNT initial yarn 85in FIG. 8 is shown being tangential to the spool, but the initial yarnmay also encounter the spool at any angle up to and including beingnormal to the spool as discussed above. The cross-sections in FIG. 8relate to the prototype embodiment, and other embodiments may usealternative cross-sectional shapes and geometries.

FIGS. 9A, 9B, and 9C show images of sections of BNNT initial yarnsproduced during experimental runs using a prototype embodimentapparatus. The images are for an area of 2.3×1.7 mm. The BNNT initialyarns extend beyond the image field. The structures visible in theimages range from about 1 micron to about 2,000 microns, and are theBNNT strand and BNNT initial yarn structures, as opposed to BNNT fibersthat are too small to be seen on the magnifications used for thesefigures.

FIG. 9A shows a section of BNNT initial yarn with a slight indication ofthe internal strand structure visible near the middle of the image. TheBNNT initial yarn width is about 90 microns, which is slightly smallerthan the BNNT initial yarn collected on the spool. The width change is aresult of the slight stretching in extracting the BNNT initial yarn fromthe spool to position the BNNT initial yarn under a microscope formagnification and photography. FIG. 9B shows a section of BNNT initialyarn that has been stretched and spread apart, making the internalstrand structure visible. FIG. 9C shows multiple overlapping BNNTinitial yarns. Portions of the internal strand structure are visible.

The following paragraphs discuss the process used to spool BNNT initialyarn in embodiments of the apparatus and methods described herein. Inthe embodiment shown in FIG. 10A, the BNNT initial yarn is spooled ontoa spool at location D of FIG. 6. The spool may be rotated at an angularspeed such that the speed of the BNNT initial yarn matches thetangential speed on the spool. The spool may be made of a metal such assteel, copper or aluminum. BNNT is an electrical insulator withdielectric properties, and therefore BNNT initial yarn iselectrostatically attracted to a metal spool. The electrostaticattraction is useful for drawing the BNNT initial yarn to the spool,particularly with respect to the initial portion of a material afterstart-up.

The spool may be of sufficient diameter that the BNNT initial yarn staysin contact with the spool during winding. The first portion of BNNTinitial yarn is electrically attracted to the spool, and subsequentportions of BNNT initial yarn are both electrically attracted to otherBNNT initial yarn and held by slight tension as described elsewhereherein.

In embodiments, the cross section of the chamber interior at location Cand location D of FIG. 6, with the BNNT initial yarn and nitrogen gas,may be designed or modified such that the BNNT initial yarn is keptunder a very slight tension and the speed of the nitrogen gas matchesonto the speed of the spool. The chamber itself may be sized and shapedto encourage this behavior, and dampers may be used as described above.For example, the cross-sectional geometries of locations C and D may bemanaged such that the material flows are laminar, and any local backdrafts do not interfere with the movement of the BNNT initial yarn.

The BNNT initial yarn may be wound onto the spool as shown in FIG. 10A.In embodiments, the spool may be located at location D in the apparatus,inside the chamber and operating under the same operating pressure.Alternatively, a pressure differential system may be used that allowsBNNT initial yarn to flow outside of the pressurized chamber, and towarda spool in an atmospheric environment. In addition to rotating, thespool may be moved back and forth along its axis (i.e., an axisperpendicular to the direction of the BNNT initial yarn approaching thespook). This oscillating movement assists with distributing the BNNTinitial yarn across the spool, forming a crossing pattern on the spoolas shown in FIG. 10A.

Numerous spooling techniques may be used to simplify the removal of BNNTinitial yarn from the spool without damaging or destroying the BNNTinitial yard. For example, BNNT initial yarn from one circuit around thespool may be separated by the diameter of at least one BNNT initialyarn, from adjacent yarns. Additionally, a BNNT initial yarn may bespooled at an angle relative to any other BNNT initial yarns that itcontacts, by at least 5 degrees or more. These separations and anglesare controlled by the relative speeds of spool rotation and the cyclicmovement back and forth of the spool along its axis.

The BNNT initial yarn may be, in some embodiments, optically observablealong the entire length of the path downstream of self-assembly. TheBNNT initial yarn position and speed may be measured by cameras, forexample, although other sensor technologies may be used. Feedback fromthe cameras or other sensors may be provided a controller operating oneor more of (1) the angular rotation speed of the spool, (2) the heatsource power as it varies the amount of heat and thereby the convectivemotion of the nitrogen gas carrying the BNNT initial yarn, (3) thecooling of the nitrogen gas, and (4) the settings of the dampers inlocations B, C, D and E. Variables (3) and (4) may also vary theconvective flow, and thereby vary the tension in the BNNT initial yarn.These parameters allow for providing slight tension in the BNNT initialyarn, matching the BNNT initial yarn speed approaching the spool to thespool's tangential speed, assists with continued BNNT initial yarnspooling, and allows for adjustments to any changes in the boronfeedstock, such as, for example in a continuous process, the boron ballsize during evaporation and subsequent replenishment by the boronfeeding mechanism.

The following discussion relates to the preparation of BNNT yarns fromBNNT initial yarns. After collection using an apparatus as describedabove, BNNT initial yarn collected on a spool may be de-spooled orremoved. The process may be mechanical or manual. The de-spooled BNNTinitial yarn may be re-spooled onto a new spool to form BNNT yarn asshown in FIGS. 10B and 10C. Composites and coatings may be applied tothe BNNT initial yarn during re-spooling to form coated BNNT yarn.Composites and coatings such as ceramics, polymers and metals, forexample, may more effectively bind to the BNNT fibers at the molecularlevel when applied during re-spooling and BNNT yarn formation. FIG. 10Bshows a re-spooling process in which BNNT initial yarn from a singleinitial spool is re-spooled on a new spool. The BNNT initial yarn may betwisted during the re-spooling process. Alternatively, as shown in FIG.10B, the new spool may be rotated during re-spooling to cause the BNNTinitial yarn to twist during spooling.

In some embodiments, multiple spools of BNNT initial yarn may be used togenerate BNNT yarn. FIG. 10C shows multiple spools of BNNT initial yarnsbeing de-spooled from the initial spool, pulled through guidestructures, and re-spooled together on a new spool. The initial spoolsmay be rotated, to cause the individual BNNT initial yarns to twist, andthe new spool may rotate, to twist the individual BNNT initial yarnstogether and form a new multi-BNNT initial yarn into a BNNT yarn.

Twisting the BNNT yarn from the initial spool increases the density ofBNNT strands, i.e. number of strands per unit of cross sectional area.Twisting BNNT strands and BNNT initial yarns may shorten the BNNT yarn,due to the helical shape caused by the twisting process. Also, thetwisting process may lengthen the yarn by enhancing BNNT strandalignment, particularly if the twisting and re-spooling process occursunder an increased tension in the BNNT initial yarn. Varying theseconditions varies the properties of the final BNNT yarn such asstrength, thermal conductivity and bonding to compositing and coatingmaterials.

The methods described herein may be used to produce closely-aligned BNNTfibers, BNNT strands and BNNT initial yarns. The close alignmentprovides significant physical properties. For example, BNNT initialyarns and BNNT yarns made as described herein may have both enhancedstrength and enhanced thermal conductivity. The enhanced strengthresults from the close alignment of the BNNT fibers, thereby providingenhanced bonding at the molecular level of the multiple BNNT fibers.Enhanced thermal conductivity, particularly in the direction of thelength, direction of alignment, of the BNNT yarn results from the closealignment of the BNNT fibers and BNNT strands. The close alignmentenhances phonon coupling or transfer at the molecular level between themultiple BNNT fibers.

The BNNT initial yarn and BNNT yarn, including weaves and ropes made ofBNNT initial yarn and BNNT yarn, may be composited with materials suchas ceramics, metals and polymers. The compositing and/or coating maytake place at any location in the process(es) of producing the BNNTinitial yarn, BNNT yarn, BNNT weaves and/or BNNT ropes, including forexample where the BNNT initial yarn forms, where it is spooled, where itis spun into BNNT yarn, and/or where it is unwound and woven into weavesand/or ropes. The method of compositing will depend on, among otherfactors, the type of material(s) being composited, and where in the BNNTmaterial process the compositing occurs. FIG. 11A-11E show examples ofBNNT initial yarn 1101, loosely wound BNNT yarn 1102, loosely woundmultiple BNNT initial yarns into a BNNT yarn and/or BNNT rope 1103, theBNNT yarn and/or rope in 1103 that has been stretched 1104 and threeseparate BNNT yarns woven into a simple flat weave 1105. These are onlya few representative options for combining BNNT initial yarns into BNNTyarns, BNNT ropes, and/or BNNT weaves, for example the number of BNNTyarns woven into a weave could be a larger number, i.e. 4, 5, 6, 7, etc.

As shown in FIGS. 12A-12C, the BNNT initial yarn, BNNT yarn, BNNT rope,and/or BNNT weave (these structures are generally referenced as “BNNTstructures” hereinafter) can be composited with other materials, such asceramics, metals, and polymers, including combinations thereof, alongany length of the BNNT structure. FIGS. 12A-12C show an embodiment inwhich a first compositing material has been added to a BNNT structure1201, and the initial composite locations are periodic along the lengthof the BNNT structure 1201. Of course, the compositing pattern may beany variation suitable for the intended application. FIGS. 12A-12C alsoshow a second embodiment in which a second compositing material has beenadded at the un-composited locations along the BNNT structure 1201 toform a multi-coated BNNT structure 1202. The second compositing materialmay be added during the same step as the first compositing material, ormay be added after initial compositing action. For example, the firstcompositing material could be a metal such as aluminum, and the secondcompositing material could be a ceramic or polymer. The compositingprocess may be continued such that three or more compositing materialshave been composited on the BNNT structure. The plurality of compositingmaterials may be periodically located, or the sequence and patterningmay be selected to be suitable for the intended use(s). The sequence ofcomposited materials may be determined by the combination oftemperature, chemical reactions, etc. that are required to integrate aspecific composited material. For example, a spark sintering processmight be utilized to integrate/composite a ceramic, and then a hightemperature laser process might be utilized to integrate/composite ametal, or a UV light or plasma beam interacting with the BNNT yarn andan epoxy might be utilized to integrate/composite the epoxy into theBNNT structure.

Finally, FIGS. 12A-12C show that in some instances the composited BNNTstructure 1203 may be stretched at any one or more stages in theprocess. The stretching step enhances the interaction between BNNTmaterial components via enhanced overlap of the multiple BNNT fibersthat make up the BNNT strands that in turn make up the BNNT initialyarn, BNNT yarn, BNNT rope and/or BNNT weave. The enhanced interactionsenhance the thermal conductivity and the strength of the finalcomposite.

The final composited BNNT initial yarn, BNNT yarn, BNNT rope, and/orBNNT weave may be cut into multiple pieces such that one or more of thepieces or segments can be utilized. For example, a piece that is acomposite of BNNT yarn with aluminum and a polymer might be placed in adiode, LED, transistor, or Integrated Circuit (IC) to enhance thethermal conductivity, and thereby lower the junction temperature(s), andthereby enhance the lifetime and efficiency of the electronic componentMultiple segments of a BNNT structure can be integrated into a singleelectronic component if desired.

The processes for compositing the BNNT initial yarn, BNNT yarn, BNNTrope, and/or BNNT weave with ceramics, metals and/or ceramics can befully automated and integrated into the same processes used for creationof the BNNT initial yarn, BNNT yarn, BNNT rope and/or BNNT weave.

Experimental results for prototype apparatus embodiments for formingBNNT fibers, BNNT strands and BNNT initial yarns are shown in FIGS. 13,14, and 15. In FIG. 13 the growth zone (Zone 1) is generally locatedwhere BNNT fibers are self-assembled, just above the boron melt. In thisinstance, the BNNT fibers directly feed into a “cotton ball” of BNNT,i.e. BNNT strands are beginning to form when the BNNT fibers areembedded into the BNNT “cotton ball.” FIGS. 14 and 15 show BNNT initialyarns collected on spools. In the embodiment utilized in FIG. 14, thespool was of variable diameter so as to be able to more easilyinvestigate the velocity of the BNNT initial yarn. In the embodimentutilized in FIG. 15 a constant diameter spool was utilized. The spoolsshown in FIGS. 14 and 15 were placed sufficiently far from the boronmelt such that all three zones were present in the process, i.e. Zone 1for BNNT fiber growth/self-assembly (as shown in FIG. 13), aninterference zone or Zone 2 for the BNNT strand formation, and a stretchzone or Zone 3 for the BNNT initial yarn formation. The images shown inFIG. 9 were taken from material collected by the embodiment shown inFIG. 15.

The principles described herein may be embodied in other specific formswithout departing from the spirit or characteristics thereof. Thepresent embodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the claims of the application rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the approach. Asused herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

What is claimed is:
 1. A process for producing aligned boron nitridenanotube (BNNT) fibers, the process comprising: feeding nitrogen gas toa chamber at an elevated pressure and flowing the gas through thechamber in a first direction; thermally exciting a boron feedstock inthe chamber to form boron micro-droplets downstream of the boronfeedstock in the first direction; establishing a heat distributionprofile to form a BNNT growth zone in the chamber downstream of theboron feedstock in the first direction, wherein BNNTs self-assemble fromthe boron micro-droplets and nitrogen gas in the growth zone and flow inthe first direction; establishing a shear force profile in the chamberto align BNNT fibers flowing from the growth zone in the firstdirection; and applying at least one of a composite and a coating to atleast a portion of the BNNTs.
 2. The process of claim 1, whereinthermally exciting the boron feedstock comprises forming a boron melt.3. The process of claim 1, further comprising forming a BNNTinterference zone in the chamber downstream of the BNNT growth zone, inwhich BNNT fibers flowing in the first direction form BNNT strands. 4.The process of claim 3, wherein applying at least one of a composite anda coating to at least a portion of the BNNTs occurs in the BNNTinterference zone.
 5. The process of claim 3, further comprising forminga BNNT stretch zone in the chamber downstream of the BNNT interferencezone, in which at least a portion of the BNNT strands flowing from theinterference zone increase in length and decrease in diameter.
 6. Theprocess of claim 5, wherein applying at least one of a composite and acoating to at least a portion of the BNNTs occurs in the BNNT stretchzone.
 7. The process of claim 1, wherein the at least one of a compositeand a coating comprises at least one of a ceramic, a polymer, and ametal.
 8. The process of claim 1, further comprising collecting BNNTs ona spool to form a BNNT yarn.
 9. The process of claim 8, wherein applyingat least one of a composite and a coating to at least a portion of theBNNTs occurs prior to collecting BNNTs on the spool.
 10. The process ofclaim 8, wherein, prior to collecting BNNTs on the spool, the directionof the aligned BNNT fibers flowing from the growth zone is changed fromthe first direction.
 11. The process of claim 8, wherein the alignedBNNT fibers flowing from the growth zone have an initial yarn speed andthe spool has a tangential spooling speed, and the initial yard speed isabout the same as the tangential spooling speed.
 12. The process ofclaim 8, further comprising de-spooling the BNNT yarn, and re-spoolingthe BNNT yarn onto a second spool.
 13. The process of claim 12, whereinapplying at least one of a composite and a coating to at least a portionof the BNNTs occurs prior to the re-spooling.
 14. The process of claim1, further comprising applying at least one of a second composite and asecond coating to at least a portion of the BNNTs.
 15. A process forproducing aligned boron nitride nanotube (BNNT) fibers, the processcomprising: feeding nitrogen gas to a chamber at an elevated pressureand flowing the gas through the chamber in a first direction; thermallyexciting a boron feedstock in the chamber to form boron micro-dropletsdownstream of the boron feedstock in the first direction; establishing aheat distribution profile to form a BNNT growth zone in the chamberdownstream of the boron feedstock in the first direction, wherein BNNTsself-assemble from the boron micro-droplets and nitrogen gas in thegrowth zone and flow in the first direction; establishing a shear forceprofile in the chamber to align BNNT fibers flowing from the growth zonein the first direction; and forming BNNT initial yarn downstream of thegrowth zone.
 16. The process of claim 15, further comprising applying atleast one of a composite and a coating to at least a portion of theBNNTs.
 17. The process of claim 15, further comprising at least one oftwisting the BNNT initial yarn, braiding the BNNT initial yarn, andweaving the BNNT initial yarn.
 18. The process of claim 18, furthercomprising applying at least one of a composite and a coating to atleast a portion of the BNNTs.
 19. The process of claim 16, furthercomprising applying at least one of a second composite and a secondcoating to at least a portion of the BNNTs.
 20. The process of claim 16,wherein the applying at least one of a composite and a coating to atleast a portion of the BNNTs comprises at least one of patterning the atleast one of a composite and a coating, and sequencing the at least oneof a composite and a coating.